X chromosome activity different in every woman

UNEXPECTED and baffling variations between individual women have emerged from a study of the X chromosome.

While men have one X and one Y chromosome, women have two X chromosomes. If all the genes on both X chromosomes were active, women would get an overdose of the proteins these genes code for. To prevent this, every cell in the early female mammalian embryo switches off one of its X chromosomes, which then remains silent in all the descendants of that cell - a process called X inactivation.

However, while some cells switch off the X inherited from the father, others switch off the X chromosome from the mother. So most women are a mixture of two different cell populations, each of which is expressing genes on a different X chromosome (New Scientist, 10 May 2003, p 42. Occasionally this can have consequences. For instance, if one X chromosome carries a ...

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